1. Field of the Invention
The present invention relates to a harmonic suppressing device for suppressing higher harmonics generated by higher harmonic current sources provided in the power transmission system, power distributing system and/or power circuit of various industrial equipments and, more specifically, to a harmonic suppressing device for suppressing higher harmonics which are generated by a rectifier such as a thyristor, a nonlinear load equipment such as an arc furnace or the saturation of the core of a transformer, and cause various troubles such as noise generation, the overheat of the components of the equipments, television disturbance and faults in control systems.
2. Description of the Prior Art
In recent years, higher harmonic troubles attributable particularly to semiconductor rectifiers employing thyristors or power converters comprising thyristors such as, for example, cycloconverters, occur frequently in power transmission systems and power distributing systems. Various measures have been taken with the progress of semiconductor techniques to eliminate such higher harmonic troubles.
FIGS. 1A, 1B and 2 are a circuit diagram, a circuit diagram of an equivalent circuit and a graph showing the characteristics of the equivalent circuit, respectively, of an exemplary conventional receiving and distributing equipment having the possibility of higher harmonic generation. Referring to FIG. 1A, the receiving and distributing equipment comprises an AC power source 1 such as, for example, an AC generator, a higher harmonic current source 2 which generates a higher harmonic current I.sub.H when driven by power supplied thereto from the AC power source 1, namely, a load power converter such as, for example, a cycloconverter, and a passive harmonic filtering device 3 for filtering higher harmonic currents generated by the higher harmonic current source 2 such as, for example, a passive filter. The power source 1 and a line 4 produce a generator impedance and a line impedance, respectively. The sum of the generator impedance and the line impedance is indicated by an impedance (represented by a parameter "L.sub.PS ")5. The harmonic filtering device 3 such as, for example, a passive filter, is an LC tuning filter comprising, for example, an inductance 6 (represented by a parameter "L.sub.f ") and a capacitance 7 (represented by a parameter "C.sub.f ")
The receiving and distributing equipment is represented by an equivalent circuit shown in FIG. 1B. The impedance characteristics Z of L.sub.PS on the side of the load in FIG. 1B, namely, the higher harmonic current source 2, is shown in FIG. 2.
The operation of the receiving and distributing equipment thus constituted will be described hereinafter. Suppose that the frequency of a higher harmonic current I.sub.H produced by the higher harmonic current source 2 is f.sub.R. Higher harmonics attributable to the higher harmonic current I.sub.H are eliminated by the harmonic filtering device 3 by making the tuned frequency coincide with the frequency f.sub.R on the basis of the following Expression. EQU Tuned frequency=f.sub.R =1/2.pi..sqroot.L.sub.f .multidot.C.sub.f ( 1)
where f.sub.R is the frequency of the higher harmonic current I.sub.H, L.sub.f is the value of inductance of the inductance 6 of the harmonic filtering device 3, and C.sub.f is the capacitance of the capacitor 7 of the harmonic filtering device 3. Thus, the inductance L.sub.f and the capacitance C.sub.f are determined from Expression (1).
In FIG. 1A, designated by I.sub.C and I.sub.S are a current that flows into the harmonic filtering device 3 and a current that flows out from the harmonic filtering device 3 into the AC power source 1, respectively.
In the harmonic filtering device 3, since the tuned frequency is adjusted to f.sub.R to filter higher harmonic currents among the higher harmonic current I.sub.H, the current I.sub.S that flows into the AC power source 1 can be suppressed when the variation of the frequency f.sub.R of the higher harmonic current I.sub.H is moderate and is maintained substantially at a fixed level. However, when the frequency f.sub.R is variable, the higher harmonic current I.sub.S that flows into the AC power source 1 increases as the frequency f.sub.R approaches resonance frequency (antiresonance frequency) f.sub.AR between the impedance L.sub.PS and the passive filter, namely, the harmonic filtering device 3 in the foregoing equipment. Such a relation is expressed by EQU f.sub.AR =1/2.pi..sqroot.(L.sub.f +L.sub.ps).multidot.C.sub.f ( 2)
Such a passive harmonic filtering device is unable to suppress higher harmonic currents effectively when the range of variation of the frequency f.sub.R including the antiresonance frequency f.sub.AR is wide.
Another exemplary harmonic suppressor connected to a three-phase AC power source among those which suppress higher harmonics on the above-mentioned principle of operation will be described hereinafter.
FIGS. 3 and 4 illustrate a conventional harmonic suppressor disclosed in Nisshin Denki Giho (technical report), issued by Nisshin Denki (electric apparatus) Co., Ltd., Jan., 1979. In FIGS. 3 and 4, parts similar to or corresponding to those previously described with reference to FIGS. 1A and 1B are denoted by the same reference characters. Referring to FIG. 3, a distributing equipment comprises a three-phase AC power source 1 comprising AC power sources 1a, 1b and 1c, a higher harmonic current source 2 as a load such as, for example, a load cycloconverter, and, for example, a passive harmonic filtering device 3. An impedance 5 including inductive impedances 5a, 5b and 5c is produced in wires 4a, 4b and 4c forming a line 4 interconnecting the AC power sources, 1a, 1b and 1c and the higher harmonic current source 2. The harmonic filtering device 3 comprises a reactor 6, a capacitor 7 and a resistor 8. The reactor 6 includes reactors 6a, 6b and 6c respectively for the wires 4a, 4b and 4c, the capacitor 7 includes capacitors 7a, 7b and 7c respectively for the wires 4a, 4b and 4c, while the resistor 8 includes resistances 8a, 8b and 8c respectively for the wires 4a, 4b and 4c.
The manner of operation of the harmonic filtering device 3 will be described hereinafter. When the higher harmonic current source 2 is, for example, a 12-phase cycloconverter, the main components of the higher harmonic current I.sub.H produced by the cycloconverter are 11th and 13th higher harmonics. Accordingly, the passive harmonic filtering device 3 is designed so as to be tuned at a frequency substantially corresponding to that of the 11th harmonic. Since the impedance 5 of the line 4, in general, is an inductive impedance as mentioned above, a ratio I.sub.S /I.sub.H, where I.sub.H is a higher harmonic current produced in the circuit of FIG. 3 and I.sub.S a higher harmonic current that flows into the power source 1, has characteristics as shown in FIG. 4. The most part of the higher harmonic current I.sub.H is absorbed by the reactor 6 and capacitor 7 of the passive filter at a tuning point A near the 11th higher harmonic, and hence curve representing the variation of the ratio I.sub.S /I.sub.H has a minimum at the point A as shown in FIG. 4. On the contrary, the curve of the ratio I.sub.S /I.sub.H has a maximum at a point B due to antiresonance between the harmonic filtering device 3 and the power source 1, and the higher harmonic current increases. The degree of the increase of the higher harmonic current is dependent on the resistance of the resistor 8. When the resistance of the resistor 8 is small, the higher harmonic current is increased by a magnification in the range of ten to twenty, and thereby the extraordinarily large higher harmonic current I.sub.S flowing into the power source has injurious influences upon the power supply system.
The conventional harmonic suppressor thus constituted inevitably has an antiresonance point, where the very large magnification of the higher harmonic current occurs. Accordingly, the passive filter of the harmonic suppressor is designed so that the antiresonance point thereof such as the point B shown in FIG. 4 will not coincide with the orders of higher harmonics to obviate the injurious influences of the magnified higher harmonic currents upon the power supply system.
However, when the higher harmonic current source 2 is a cycloconverter, the order of a higher harmonic current necessarily coincides with the antiresonance point since the frequency of the generated higher harmonic current varies according to the output frequency of the cycloconverter as represented by Expression (3) shown hereinafter. Accordingly, it is impossible to obviate the occurrence of the very large magnification of the higher harmonic current.
The frequency f.sub.n of an nth higher harmonic current is expressed by EQU f.sub.n =(6m.+-.1)f.sub.1 .+-.6k.multidot.f.sub.0 ( 3)
where n is the order of a higher harmonic current, f.sub.1 is the frequency of a fundamental wave, f.sub.0 is the output frequency of the cycloconverter, and m and k are integral numbers.
For example, when m=1, f.sub.1 =60 Hz, f.sub.0 =0 to 10 Hz, and k=1, the frequency f.sub.5 of the fifth higher harmonic current varies in the range of 240 to 360 Hz, namely, in the range of the fourth to sixth higher harmonic, while the frequency f.sub.7 of the seventh higher harmonic current varies in the range of 360 to 480 Hz, namely, in the range of the sixth to the eighth higher harmonic, so that the frequency varies continuously in the range of the fourth to the eighth higher harmonic when both the fifth and seventh higher harmonic currents are taken into consideration. Therefore, a higher harmonic current of an order which coincides with the antiresonance point B (FIG. 4) is produced inevitably.
In the 12-phase cycloconverter, higher harmonic currents of orders above the fifth are produced and, theoretically, the fifth higher harmonic current or the seventh higher harmonic current cannot be produced. However, in practice, the fifth and seventh higher harmonic currents are produced due to unbalance between the six phases, and the fifth and seventh higher harmonic currents are magnified by resonance.
Thus, the conventional passive filter is unable to avoid the magnification of higher harmonic currents at the antiresonance point when the frequency of the higher harmonic current is variable as the frequency of higher harmonic current produced by a cycloconverter, and hence such a conventional passive filter is unable to eliminate injurious influences on the associated system.
Furthermore, in the conventional passive filter, it is necessary to increase the resistance of the resistor 8 to reduce the ratio of higher harmonic magnification. However, although the ratio of higher harmonic magnification at the antiresonance point B' is reduced when the resistance of the reactor 8 is increased, the higher harmonic absorption ratio near the resonance point A' is reduced and electrical loss across the resistor 8 increases.